What an accumulator does
When an automatic selects a new gear, fluid pressure applies a clutch or band. If that pressure arrived all at once, every shift would bang. Accumulators soften it: they absorb some of the fluid at first, so pressure rises over a short period instead of instantly.
In some gearboxes, the Toyota A340 being a well-known one, the back sides of all the accumulators are joined and fed by a single pressure regulator. Control that back-pressure and you control how fast every shift applies.
| Back-pressure | Shift feel | Trade-off |
|---|---|---|
| Higher | Quicker, firmer shifts | Harsher on driveline and passengers |
| Lower | Slower, smoother shifts | Smoothness comes from clutch and band slip, and heavy slip under load wears them out |
Soft shifts feel nice, but they are soft because the friction elements are slipping. On a powerful car, don't chase smoothness at high load, or you will shorten the life of the transmission.
Setting it up
Output frequency
Sets the PWM frequency for the regulator solenoid. The A340E runs at 300 Hz. For other transmissions, use the frequency from its service information or base map.
Duty cycle table
This is where shift feel is actually tuned. If Haltech offers a base map for your transmission, it will already have this table filled in. Using a gearbox with no base map? The A340 base map is a sensible starting point: import just its Accumulator Back-Pressure Duty Cycle table rather than the whole map. See base map basics if you haven't imported a map before.
Pay attention to direction. Most transmissions use a pressure relief style valve, so more duty means less pressure. Adding duty will make shifts softer, not firmer. Confirm how your unit responds before making big changes.
Wiring
Assign the Accumulator Back-pressure Control Solenoid to an output pin in the wiring node.
How we approach it
- Start from a base map value, never a blank table.
- Change in small steps and road test each change under light and heavy throttle.
- Log throttle, gear and duty together so you can tie each shift to what was commanded.
- Favor firmer shifts at high load, where slip causes wear.
Accumulator control works hand in hand with line pressure control, and both sit under the Transmission Control function. For lockup, see torque converter control.
